1. Field of the Invention
The present invention relates to a communication system and more particularly to a photonic arbitrary waveform modem for use in various applications including secure multiple access communication systems which utilizes bipolar encoding to provide true orthogonal coding to reduce co-channel interference.
2. Description of the Prior Art
Multiple access communication systems are generally known in the art and are used in various applications, such as cellular telephone communication systems. Various multiple access communication systems are known, for example, frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA) systems are known. In FDMA communication systems, the frequency band is partitioned into a number of channels. Each user is temporarily assigned a channel, for example, for the duration of a call. In TDMA communication systems, each user is assigned a time slot in a frame and each call is time division multiplexed according to the user's assigned time slot.
In order to further improve the system capacity, CDMA communication systems have been developed. Examples of CDMA communication systems are disclosed in U.S. Pat. Nos. 4,494,228; 4,901,307; 5,103,459; 6,185,246; and commonly-owned U.S. Pat. No. 6,167,024. Such CDMA communication systems are also disclosed in “Synthesis and Demonstration of High Speed, Bandwidth Efficient Optical Code Division Multiple Access (CDMA) Tested at 1 Gb/s Throughput” by Mendez, et al. IEEE Photonics Technology Letters, Vol. 6, No. 9, September 1994, pages 1146-1149, all hereby incorporated by reference.
In such CDMA communication systems, a code, known as a Walsh code, is assigned to a user at the beginning of each communication and multiplied by each databit in the signal to be transmitted (i.e. digital bit stream), effectively spreading the signal over a wider frequency band, thus forming a direct sequence spread spectrum communication system. In such CDMA communication systems, all users effectively use the same timeslot and frequency band. Interference between users is prevented by selection of orthogonal Walsh codes. An advantage of spread spectrum communication techniques, such as CDMA, is the relatively low probability that the communication signals will be intercepted and detected.
In order to further increase the security in military communication systems, an arbitrary waveform modem has been developed that is characterized by non-periodic or chaotic waveforms. The arbitrary waveform modem is described in detail in commonly-owned copending U.S. patent application Ser. No. 09/120,851, filed on Jul. 22, 1998, hereby incorporated by reference and illustrated in FIGS. 1 and 2. In particular, FIG. 1 illustrates an arbitrary waveform modem transmitter, while FIG. 2 illustrates an arbitrary waveform receiver. As will be discussed in more detail below, the arbitrary waveform modem provides the ability to select infinitely variable tap spacings through the use of fiber optic Bragg gratings. It also enables the generation of an arbitrary waveform with relatively long symbol times, large bandwidths and non-uniform tap spacing which allows the modulated waveform to be any shape necessary. Since the modem can generate non-uniform tap spacings, the phasing of the chip, baud or symbol can be arbitrarily set to any value to produce a waveform characteristic that degenerates the rate line spectral components making interception improbable. Additionally, co-channel interference is reduced because the cross-correlation between orthogonally selected waveforms diminishes.
In general, the photonic arbitrary waveform modem modulates a signal waveform (e.g. digital bit stream) onto an optical carrier that is derived from a broadband source, for example, a super-luminescent diode (SLD). The various optical frequency components or chips are time delayed by differing amounts by a set of narrow band Bragg grating filters before being transmitted. At the receiver, another set of Bragg grating filters temporally realigns all of the frequency chips providing an increase in the signal-to-noise ratio that is equal to the number of chips in the code which enables recovery of the signals.
One major shortcoming of the photonic arbitrary waveform modem is that it relies on the direct summation of optical intensities which restricts the system to a set of unipolar codes (i.e. each tap weight can take on a value of 0 or 1). For a given number of wavelength taps, this places a limitation on the number of codes that can be generated that offer a high degree of orthogonality while providing reasonable processing gain. For example, for a case for which there are 16 taps, the Mendez article, discussed above, indicates that an optimum set of 16 pseudo-orthogonal code words can be constructed by utilizing various combinations of 5 taps. These codes produce an autocorrelation value of 5 and cross correlation values of either 1 or 2. Thus, there is a need for a photonic arbitrary waveform modem which increases the number of codes that can be generated to provide optimum efficiency of spectral occupancy while at the same time provide low co-channel interference modulation characteristics.